1. Field of the Invention
The invention pertains to diindenothiophene derivatives that can be used as dyes for dye-sensitized solar cells.
2. Description of the Prior Art
Rapid technological and economic advancements have fostered ever-increasing demand for energy, resulting in declining reserves of petroleum, natural gas, coal, and the like. Meeting this energy demand requires development of new resources. Solar energy is one of the most attractive and important resources for such further development as it generates little or no pollution. Of the many types of solar cells that have been developed to meet this need, dye-sensitized solar cell (DSSC) has the advantages of low manufacturing cost and potential for development.
DSSC was first developed in 1976 by Hiroshi Tsubomura et al. using porous ZnO as an electrode, and achieved a power conversion efficiency of 2.5%. The photoelectric conversion efficiency of DSSC was not substantially increased until 1991, when a Swiss team led by M. Grätzel achieved an efficiency of 7.1˜7.9%, opening the door for future commercialization. The DSSC developed by M. Grätzel's team in Switzerland utilized an anode produced by coating TiO2 nano-crystals onto indium-tin-oxide (ITO) glass and had Ru-complex dyes (typically, N3 and N719) absorbed by the porous structure of the porous film formed from the TiO2 nano-crystals to absorb visible light as well as a cathode of a conductive glass plated with platinum, and utilized I−/I3− solution as an electrolyte for providing the oxidation-reduction reaction necessary for the cell. N3 and N719 have the following structures:

As mentioned above, a dye-sensitized solar cell mainly consists of five parts: a cathode and anode for providing current paths, a TiO2 semiconductor layer for use as an electron transport layer, a dye layer, and an electrolyte for transporting holes. All the materials and the interfacial structures of these parts will play a role in the performance of solar cells. In particular, the dye used in the dye layer is crucial to solar cell efficiency. Therefore, the search for suitable dyes to enhance solar cell efficiency has become an important objective of solar cell development.
Dyes for dye-sensitized solar cells are normally classified into two types: those containing the above-mentioned Ru-complexes, and those comprised of organic molecules. Compared to metal complexes, organic molecules have the following advantages: (1) higher molar absorption coefficient; (2) greater flexibility in structural design and possible range of light absorption; and (3) absence of noble metals, thus eliminating dependence on limited and environmentally hazardous material sources while also reducing production costs.
In 1996, Michael Grätzel employed the natural substance coumarin as a main dye ingredient in a DSSC. However, only 0.9% photoelectric conversion efficiency was achieved, probably due to the relatively narrow absorption band of coumarin preventing efficient utilization of sunlight.
Given the above, there is ongoing demand for organic molecular materials for dyes achieving greater efficiency of sunlight absorption in dye-sensitized solar cells.